I'll put it this way then, one guy wrote the story, designed the characters, we dont have his permission, and we dont have toei animations permission for using their sounds for characters, or funimations for their dub sounds, however, we do not include entire episodes, neither do we tell the story. This becomes a free advertisement for them, as its free, they arent loosing anything by letting us continue with this project, because we not only introduce people to the series but maintain its interest. We arent using enough of their product for somebody to not watch dbz, somebody isnt going to say " yeah! now i have goku saying "kame hame - ha!" on my computer, im not gonna buy that dvd now to watch the show because im happy with these 2 second sound files. However with a song, you are taking the whole of somebody's work, not advertising, or helping them in any way, because you are directly supplying people with that, and therefore there is no need for you to buy the music if we give it out for free, that leads to loss in sales, and that leads to us getting sued or told to "cease and decist(sp?) or legal action would be taken".
So, after you've thought a little and taken that all in, maybe you'll realise that a 5 year long project has 99% most likely had suggestions like this before, and if it was turned down its for a good reason.